As development of optical fiber communication technologies, higher optical fiber transmission capacities are demanded. Currently, a system transmission capacity of an optical fiber communication system is mainly improved by using a polarization multiplexing technology, which, by loading different modulation signals in two orthogonal polarization states (H polarization state and V polarization state), may double the system transmission capacity without increasing a bandwidth.
However, for a high-speed optical fiber communication system of polarization multiplexing, damages in the optical fiber links, such as polarization mode dispersion (PMD) and polarization dependent loss (PDL), brought about by some effects related to the polarization states, must be taken into account in long-haul transmission, such effects posing non-negligible influence on performance of the optical fiber communication.
The PMD in an optical fiber makes that signals are propagated forward at different speeds in the two polarization states, thereby making pulses widened and system performance degraded. And PDL in an optical fiber refers to a ratio of maximum transmission power Pmax to minimum transmission power Pmin of the optical fiber in all possible polarization states, which may be expressed as follows:
                    PDL        =                  10          ⁢                      log            10                    ⁢                                                    P                max                                            P                min                                      .                                              formula        ⁢                                  ⁢        1            
Hence, the PDL in the optical fiber will result in changes of optical power and signal to noise ratios in the two polarization states, thereby seriously affecting the performance of the polarization multiplexing system. In order to improve the performance of the polarization multiplexing system, the PDL in the optical fiber link needs to be accurately estimated.
It should be noted that the above description of the background is merely provided for clear and complete explanation of this disclosure and for easy understanding by those skilled in the art. And it should not be understood that the above technical solution is known to those skilled in the art as it is described in the background of this disclosure.